In order to purify an exhaust gas, a catalyst is disposed in an exhaust pipe. An exhaust gas sensor, such as an air/fuel ratio sensor or an oxygen sensor, is arranged upstream of the catalyst in order to control an air-fuel ratio of the exhaust gas. Based on a detected value of the exhaust gas sensor, a quantity of fuel injection is feedback controlled to obtain a target air-fuel ratio. In such a conventional system, a deterioration diagnosis of the exhaust gas sensor is conducted.
JP-A-H01-155257 shows an evaluation method of the exhaust gas sensor performance. In this method, a lean control and a rich control are interchangeably conducted. In the lean control, the air-fuel ratio is changed from a rich condition to a lean condition by varying a fuel injection quantity. In the rich control, the air-fuel ratio is changed from the lean condition to the rich condition. A response time of the exhaust gas sensor is measured in the lean control and the rich control. The response time is a time that is required for the output value of the exhaust gas sensor to be changed from a predetermined first value to a predetermined second value. The evaluation of the exhaust gas sensor performance is conducted based on the response time in the lean control and the response time in the rich control.
It is not always that a responsiveness of an air-fuel ratio sensor deteriorates in a lean direction and a rich direction equally. The responsiveness in only one direction may deteriorate. If the responsiveness deteriorates in only one direction, its effects hardly appear in a deterioration determining parameter. The deterioration determining parameter is represented by an average of a responsiveness characteristic in lean direction and a responsiveness characteristic in rich direction. A difference in deterioration determining parameter may not appear between cases where the air-fuel ratio sensor is normal and where the air-fuel ratio sensor deteriorates in only one direction.
According to inventors' experiment, as shown in FIG. 7, a large part of a dispersion of the deterioration determining parameter are overlapped with each other between the normal air-fuel ratio sensor and the air-fuel ratio sensor deteriorated in one direction of responsiveness. Hence, according to a deterioration diagnosis method in which an average of responsiveness characteristics in the lean and rich directions is used as the deterioration determining parameter, if the responsiveness of the air-fuel ratio sensor deteriorates only in one direction, such deterioration may not be detected with high accuracy.